For the macaroons, line 2 large sturdy baking sheets with non-stick baking parchment.
Whisk the egg whites in a clean bowl until stiff peaks form when the whisk is removed. Gradually whisk in the caster sugar, a spoonful at a time, until thick and glossy.
Sift the icing sugar and ground almonds into a bowl, then stir in the lemon zest and juice and a few drops of yellow food colouring. Fold in a third of the egg whites, then gradually fold in the remainder. Try not to overmix the batter - it should be smooth and glossy and when the spatula is lifted above the bowl, the mixture should fall back down in ribbons.
Spoon the macaroon mixture into a piping bag, and pipe macaroon fingers about 5cm/2in long onto the baking paper, spacing them well apart. (You will need two fingers for each macaroon, so make sure you have an even number.) Set the macaroons aside at room temperature for about 20 minutes, or until a skin forms on top and the mixture doesn't stick to your finger when touched.
Meanwhile, preheat the oven to 180C/360F/gas 4.
Bake the macaroons in the oven for 10-15 minutes, or until the macaroons have risen slightly (there should be a thin, frilled edge around the bottom, called 'feet') and are beginning to firm up. Remove from the oven and set aside to cool on the baking sheet. When cooled, carefully peel the macaroons away from the baking parchment. If they stick to the sheet, lightly wet the bottom of the parchment with water and they should come away easier.
For the lemon curd, place the caster sugar into a heatproof bowl with the eggs and egg yolks. Add the lemon zest and juice, then place the bowl over a pan of boiling water (make sure the bottom of the bowl doesn't touch the water) and whisk continuously until the mixture is thickened. Gradually whisk in the butter, a little at a time until, smooth and thick enough to coat the back of a wooden spoon. Set aside to cool.
To serve, spoon the cooled lemon curd into a piping bag. Pipe a line of lemon curd along one macaroon half, then sprinkle over a little bit of popping candy and sandwich with another macaroon half. Serve immediately.
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